An attendee tries out an M&P 15-22 Sport rifle at the Smith & Wesson booth at the 2016 National Shooting Sports Foundation's Shooting, Hunting, Outdoor Trade (SHOT) Show at the Sands Expo and Convention Center on January 19, 2016 in Las Vegas, Nevada.

After the mass shooting and killing of schoolchildren in Newtown, Connecticut, in late 2012, activists made an appeal to Wall Street money managers: sell your stock holdings of gun makers and gun sellers.

It was a similar argument activists had made to exert pressure on tobacco and big oil companies. But while their message got a sympathetic ear from several high profile pensions and consumer advocacy groups, in practice it hasn't been so easy for the finance world to dump the gun industry.

Indeed, two of the world's biggest money managers, BlackRock and Vanguard, are among the top shareholders of three of the biggest publicly traded gun manufacturers as well as some of the biggest gun retailers.

BlackRock and Vanguard are also the top two holders of big gun sellers, including Dick's Sporting Goods, with about 7 percent each, and top three along with State Street Global Advisors among Walmart holders.

They have something else in common: BlackRock and Vanguard are among the biggest managers of exchange traded and index funds, which buy all the stocks in a given index in an effort to reproduce its performance. American Outdoor, Sturm Ruger and Vista are in the Russell 2000, while Walmart is on both the S&P 500 and the Dow Jones industrial average.

Wednesday's shocking killing of at least 17 people at a high school in Florida may renew calls for gun industry divestment as much as people are again talking about ways to prevent these tragedies in the future. But gun makers have been under pressure for some time.

Remington said Monday it would seek bankruptcy protection while it reorganizes debt.

In 2015, New York City's public advocate called on TD Bank to stop providing financing to Smith & Wesson, and she asked the Securities and Exchange Commission to examine the gun maker's public disclosures about its products. She also urged the city's largest pension to sell its holdings of Walmart and Dick's Sporting Goods.

Another investor, Trinity Church of Wall Street, took Walmart to court to get it to stop selling high capacity weapons. And it has also used its position as a shareholder of Cabela's to get that retailer to stop selling certain weapons. Bass Pro recently bought Cabela's.

Investing in a way that meets social and/or environmental goals has become a big business, much of it driven by climate-impact investing that shuns the fossil fuel industry.

Last month, BlackRock CEO Larry Fink made a big splash in an open letter to chief executives of the world's largest companies, urging them to improve society.

"To prosper over time, every company must not only deliver financial performance, but also show how it makes a positive contribution to society," the letter said.

Cameron Costa | CNBC

Larry Fink

That doesn't mean BlackRock will be dropping stocks of companies that touch the gun industry, though. "As a significant provider of index-based investments, we are required to replicate the holdings of particular indices," the firm told CNBC in a statement. "Third-party providers determine which companies are included in these indices."

The fund manager does allow investors to opt out of stocks they don't want to own through socially conscious funds like its $1 billion DSI exchange-traded fund, which avoids shares of weapons makers, vice products and companies involved in nuclear energy and genetic modification.

Likewise, Vanguard has a $4 billion asset FTSE Social Index fund. In an email, Vanguard said it has 388 funds, and 95 percent of them are not invested in companies involved in gun manufacturing. Further, the majority of funds that do are indexed.

Last fall, after the mass shooting at a concert venue in Las Vegas, California State Treasurer John Chiang urged the state's big teacher and public employee pensions to sell their holdings of companies that sell assault weapons, ammunition and other firearm devices.

Those pensions are examining the issue and are expected to report on their findings this spring.